The responsivity of an organism to a painful stimulus can be affected by numerous factors including sensory and attitudinal (response bias) variables. Signal detection theory has been used recently to determine which of these variables is affected by putative analgesia treatments in humans. The purpose of the proposed research is to further establish the usefulness of a paradigm developed by the Principal Investigator for assessing pain perception in rats using signal detection theory. After training the animals on the task and investigating the effects of various parametric manipulations, the effects of morphine, pentobarbital, and d-amphetamine on the separate measures of sensory sensitivity and response bias will be compared. The effects of chronically administered morphine on these two dependent variables will also be established. The research should result in a technique which is more useful than existing tests for establishing the efficacy of possible analgesia treatments in infrahumans and lead to data which could further delineate the mechanism of analgesia tolerance and dependence.